1. Field of the Invention
The present invention relates to a process for preparing pure phosphoric acid by removing sulfate ions from phosphoric acid prepared by the wet process.
2. Description of the Prior Art
It is known that wet process phosphoric acid which is produced by sulfuric acid decomposition of phosphate rock can be extracted by an organic solvent which dissolves the phosphoric acid and has a low solubility in water. Such solvents include various alcohols, ketones, ethers, phosphoric acid esters, amines and the like. Subsequently, phosphoric acid is extracted by water from the extracted solution of phosphoric acid (hereinafter referred to as the "extraction method"). The purified phosphoric acid prepared by further purifying a crude phosphoric acid from the wet process (including one which has been pretreated) by the extraction method (including one which has been subsequently concentrated) is referred to as an "extracted phosphoric acid".
The phosphoric acid produced by the wet process contains various metal ion impurities derived from phosphate rock and sulfate ions derived from sulfuric acid. In the extraction method, the metal ion impurities can be effectively removed. However, sulfate ions are not satisfactorily removed and a large amount of them still remain in the extracted phosphoric acid. Unfortunately, the phosphoric acid used in industrial products, medicines and foods must be pure phosphoric acid having substantially no sulfate ions. For example, a content of less than 30 wt ppm as SO.sub.4 is the Japanese Industrial Standard. In order to sufficiently decrease the concentration of sulfate ions so that the phosphoric acid can be used for industrial purposes, it is necessary to remove sulfate ions either from the wet process phosphoric acid and/or the extracted phosphoric acid. It is known to remove sulfate ions as calcium sulfate from the wet process phosphoric acid by adding a calcium compound such as phosphate rock, calcium hydroxide or calcium carbonate. But calcium sulfate has a relatively high solubility. Accordingly, although a large excess of calcium compound is used, sulfate ions remain in a concentration in the range of 1000 - 2000 wt ppm as SO.sub.4. It is difficult to further lower the concentration of sulfate ions. Even when the desulfated wet process phosphoric acid is further purified by extraction, a sulfate ion concentration typically of at least 500 wt ppm as SO.sub.4 remains in the extracted phosphoric acid.
It is also known to remove sulfate ions as barium sulfate to a lower concentration by using a barium compound. However, it is not preferred to use such a method in a wet process phosphoric acid because of the following reasons.
a. The sulfate ion content is higher than that of the extracted phosphoric acid whereby consumption of the expensive barium compound is high.
b. Hydrofluorosilicic acid is also present whereby the barium compound is consumed for the formation of crystals of barium fluorosilicate.
c. The contents of both sulfate ions and hydrofluorosilicic acid varies in dependence upon fluctuations in the composition of the phosphate rock. Moreover, control of operating conditions is critical whereby the phosphoric acid may be contaminated by barium ions because of an excess addition of the barium compound.
d. Hydrofluoric acid is also present. Therefore, it is difficult to appropriately select a material for use in the filter required for separating barium sulfate. It is difficult to use stainless steel.
e. The crystals of barium sulfate which are obtained are very fine and smaller than those obtained from extracted phosphoric acid. It is theorized that this may be caused by the presence of organic material and metallic impurities. Moreover, the viscosity of the slurry is high even when the concentration of phosphoric acid is low. Accordingly, the separation of barium sulfate is difficult.